1. The interaction of two or more biological agents or forces so that their combined effect is greater than the sum of their individual effects. 2. Cooperative interaction among species, especially among the individuals and groups in an ecosystem, that creates an enhanced combined effect. 3. The theory that organisms cooperate with passage of time in the same ecosystem, mainly as a result of reciprocal altruism, so that biosocial structure and dynamics change to assure the vigor of all life forms. [Greek, from bios, life. From Greek sunergia, cooperation, from sunergos, working together.)